The lungs are located in two cavities on either side of the heart. Though similar in appearance, the lungs are not identical. Both are separated into lobes by fissures, with three lobes on the right and two on the left (Figure 1).
The lobes are further divided into segments and then into lobules, hexagonal divisions of the lungs that are the smallest subdivision visible to the naked eye.
THE RIGHT LUNG
The right lung is five centimeters shorter than the left lung to accommodate the diaphragm, which rises higher on the right side over the liver; it is also broader. The volume, the total capacity, and the weight of the right lung is greater than that of the left.
The human right lung is divided into three lobes: the superior, middle, and inferior, by two interlobular fissures. The middle lobe, the smallest lobe of the right lung, is wedge-shaped, and includes the lower part of the anterior border and the anterior part of the base of the lung. The superior and inferior lobes are similar to their counterparts on the left lung.
The oblique fissure separates the inferior lobe from the middle and superior lobes. The other fissure, the horizontal fissure, separates the superior from the middle lobe.
THE LEFT LUNG
The human left lung is smaller and narrower that the right lung, and is divided into two lobes, an upper and a lower, by the oblique fissure.
The left lung has only two lobes because of the space taken up in the left side of the chest cavity by the heart. It has a depression on the medial side of its surface called the cardiac notch, a concave impression molded to accommodate the shape of the heart (Figure 2).
Above and behind the cardiac impression is a triangular depression named the hilum, where the structures which form the root of the lung enter and leave These include the pulmonary artery, the superior and inferior pulmonary veins, lymphatic vessels, and the bronchus, which is surrounded by bronchial veins and arteries.